354 Professor HelmJwltz on the Law of the [April 12, 



living body ; only the oxidation in the living body goes on more slowly. 

 The albuminous substances would give us the same substances, and 

 also nitrogen, as if they were burnt in the fire. You may suppose, for 

 making both cases equal, that the amount of urea which is produced 

 in the body of the animal, may be changed without any very great 

 development of heat, into carbonate of ammonia, and carbonate of 

 ammonia may be burnt, and gives nitrogen, water, and carbonic acid. 

 The amount of heat which would be produced by burning urea into 

 carbonic acid and nitrogen, would be of no great value when compared 

 with the great quantity of heat which is produced by burning the fat, 

 the sugar, and the starch. Therefore we can change a certain amount 

 of food into carbonic acid, water, and nitrogen, either by burning the 

 whole in the open fire, or by giving it to living animals as food, and 

 burning afterwards only the urea. In both cases we come to the 

 same result. 



Now I have said that the conservation of force for chemical pro- 

 cesses requires a fixed amount of mechanical work, or its equivalent, to 

 be given out during this process ; and the amount is exactly the same 

 in whatever way the process may go on. And therefore we must 

 conclude that by the animal as much work must be done, must be 

 given out — the same equivalent of mechanical work — as by the 

 chemical process of burning. Now let us remark that the mechanical 

 work which is spent by an animal, and which is given to the external 

 world, consists, firstly, in heat ; and secondly, in real mechanical work. 

 We have no other forms of work, or of equivalent of work, given out 

 by living animals. If the animal is reposing, then the whole work 

 must be given out in the form of heat ; and therefore we must conclude 

 that a reposing animal must produce as much heat as would be pro- 

 duced by burning its food. A small difference would remain for the 

 urea ; we must suppose that the urea produced by the animal is also 

 burnt, and taken together with the heat immediately produced by the 

 animal itself. Now we have experiments made upon this subject by 

 the French philosophers Dulong and Desprez. They found that these 

 two quantities of heat — the one emitted by burning, the other by the 

 living animal — are nearly identical ; at least, so far as could be esta- 

 blished at that time, and with those previous researches which existed 

 at that time. The heat which is produced by burning tlie materials of the 

 food is not quite known even now. We want to have researches on the 

 heat produced by the more complicated combinations which are used as 

 food. Dulong and Desprez have calculated the heat according to the 

 theoretical supposition of Lavoisier — which supposition is nearly right, 

 but not quite right — therefore there is a little doubt as to the amount 

 of the heat, but experiments show that at least to the tenth part of that 

 heat the quantities are really equal ; and we may hope if we have 

 better researches on the heat produced by burning the food, that these 

 quantities will also be more equal than they were found to be by 

 Dulong and Desprez. 



Now if the body be not reposing, but if muscular exertion take 



